CO2-facilitated transport membrane and method for producing the same

ABSTRACT

A CO 2 -facilitated transport membrane of excellent carbon dioxide permeability and CO 2 /H 2  selectivity, which can be applied to a CO 2  permeable membrane reactor, is stably provided. The CO 2 -facilitated transport membrane is formed such that a gel layer  1  obtained by adding cesium carbonate to a polyvinyl alcohol-polyacrylic acid copolymer gel membrane is supported by a hydrophilic porous membrane  2 . More preferably, a gel layer supported by a hydrophilic porous membrane  2  is coated with hydrophilic porous membranes  3  and  4.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/864,232 (now U.S. Pat. No. 8,197,576), which is a National Phasefiling under 35 U.S.C. §371 of International Application No.PCT/JP2009/051000 filed on Jan. 22, 2009 now WO 2009/093666), whichclaims priority to Japanese Patent Application No. 2008-013722 filed onJan. 24, 2008.

TECHNICAL FIELD

The present invention relates to a CO₂-facilitated transport membraneused to separate carbon dioxide, and more particularly to aCO₂-facilitated transport membrane which can separate carbon dioxidecontained in a reformed gas for a fuel cell, containing hydrogen as amain component with high selectivity for carbon dioxide over hydrogen.

BACKGROUND ART

Heretofore, a method for selectively separating carbon dioxide haswidely been studied because of its wide application range. For example,purity of hydrogen can be improved by selectively separating carbondioxide from a reformed gas for a fuel cell. Further, it is expectedthat the progression of global warming may be suppressed by selectivelyseparating carbon dioxide which is one of causes of global warming, andstoring the separated carbon dioxide on the sea bottom.

Looking at the hydrogen production process, in a reforming system for ahydrogen station, which is currently been developed, hydrogen isproduced by reforming hydrocarbon into hydrogen and carbon monoxide (CO)through steam reforming, and reacting carbon monoxide with steam using aCO shift reaction.

In a conventional CO shift reactor, the cause for inhibition ofminiaturization and reduction of the start-up time is considered that alarge amount of a CO shift catalyst is necessary because of therestriction on chemical equilibrium of the CO shift reaction representedby (Chemical Formula 1) shown below. For example, 20 L of a reformingcatalyst is required in a 50 kW reforming system for phosphoric acidfuel cell (PAFC), whereas, an about 4-fold amount (77 L) of a CO shiftcatalyst is required. This is a large factor, which inhibitsminiaturization of the CO shift reactor and reduction of the start-uptime. The symbol “

” means a reversible reaction.CO+H₂O

CO₂+H₂  Chemical Formula 1

Therefore, when a CO shift reactor is equipped with a CO₂-facilitatedtransport membrane capable of being selectively permeated by carbondioxide and when carbon dioxide at the right side produced by the COshift reaction of Chemical Formula 1 shown above is efficiently removedout of the CO shift reactor, chemical equilibrium can be shifted to thehydrogen production side (right side) to obtain a high conversion ratioat the same reaction temperature, thus making it possible to removecarbon monoxide and carbon dioxide over limitation due to therestriction of equilibrium. This state is schematically shown in FIGS.20 and 21. FIGS. 21A and 21B respectively show each change in theconcentration of carbon monoxide and carbon dioxide along the catalystlayer length of the CO shift reactor in the case where the CO shiftreactor is equipped with a CO₂-facilitated transport membrane or not.

Since the above CO shift reactor (CO₂ permeable membrane reactor)equipped with a CO₂-facilitated transport membrane enables removal ofcarbon monoxide and carbon dioxide over limitation due to therestriction of equilibrium, it is possible to reduce a load of pressureswing adsorption (PSA) of a hydrogen station and to lower S/C(steam/carbon ratio) of the reforming reaction and CO shift, thus makingit possible to reduce the cost of the entire hydrogen station andincrease efficiency. Since higher performances (increase in SV) of theCO shift reaction can be achieved by being equipped with theCO₂-facilitated transport membrane, miniaturization of the reformingsystem and reduction of the start-up time can be achieved.

Example of the related art of the CO₂ permeable membrane reactor isdisclosed in Patent Document 1 (or Patent Document 2 having the samecontents published by the same inventors).

The reforming system proposed in Patent Documents 1 and 2 provides aCO₂-facilitated transport membrane process which is useful forpurification and water gas shift reaction (CO shift reaction) of areformed gas generated when fuels such as hydrocarbon and methanol arereformed into hydrogen for a fuel cell vehicle on the vehicle, andtypical four kinds of processes are disclosed in the same PatentDocuments. When hydrocarbon (containing methane) is used as a rawmaterial, by selectively removing carbon dioxide using a membranereactor in which a water gas shifter (CO shift reactor) is equipped witha CO₂-facilitated transport membrane, the reaction rate of carbonmonoxide is increased and the concentration of carbon monoxide isdecreased, and also purity of hydrogen thus produced is increased.Further, percentage-order carbon monoxide and carbon dioxide remainingin hydrogen produced are reacted with hydrogen in a methanator therebyconverting into methane, and thus the concentrations are decreased and adecrease in efficiency of a fuel cell due to poisoning is prevented.

In Patent Documents 1 and 2, as the CO₂-facilitated transport membrane,a hydrophilic polymer membrane of PVA (polyvinyl alcohol) containingmainly a halogenated quaternary ammonium salt ((R)₄N⁺X⁻) as a carbondioxide carrier is used. Example 6 of Patent Documents 1 and 2 disclosesa method for producing a CO₂-facilitated transport membrane formed of acomposite membrane of 50% by weight of a 49-μm thick PVA membranecontaining 50% by weight of a tetramethylammonium fluoride salt as acarbon dioxide carrier, and a porous PTFE (polytetrafluoroethylene)membrane which supports the PVA membrane, and Example 7 disclosesmembrane performances of the CO₂-facilitated transport membrane when amixed gas (25% CO₂, 75% H₂) is treated under a total pressure of 3 atmat 23° C. Regarding the membrane performances, CO₂ permeance R_(CO2) is7.2 GPU (=2.4×10⁻⁶ mol/(m²·s·kPa)) and CO₂/H₂ selectivity is 19.

Patent Document 3 shown below discloses, as a CO₂-facilitated transportmembrane, a CO₂ absorbent formed by cesium carbonate in combination withamino acid.

The method for producing a CO₂-facilitated transport membrane describedin Patent Document 3 is as follows. First, a commercially availableamino acid is added to an aqueous solution of cesium carbonate so as toobtain a predetermined concentration, followed by well stirring toprepare an aqueous mixed solution. A gel-coated surface of a gel-coatedporous PTFE membrane (47Φ) is then immersed in the prepared mixedsolution for 30 minutes or more, and the membrane is slowly pulled up. Asilicone membrane is placed on a sintered metal (for the purpose ofpreventing the permeation side from being wetted with the solution) andthe above hydrogel membrane (47 mmΦ) is placed thereon, followed bysealing through covering with a cell with a silicone packing. A feed gasis allowed to flow at a rate of 50 cc/min over the CO₂-facilitatedtransport membrane thus produced, and the pressure of the lower side ofthe membrane is reduced to about 40 torr by evacuating the lower side.

In Example 4 of Patent Document 3, when a CO₂-facilitated transportmembrane formed by cesium carbonate and 2,3-diaminopropionic acidhydrochloride at each molar concentration of 4 (mol/kg) is used, a CO₂permeation rate is 1.1 (10⁻⁴ cm³(STP)/cm²·s·cmHg) and a CO₂/N₂separation factor is 300 under the temperature of 25° C. Since the CO₂permeance R_(CO2) is defined by a permeation rate per pressuredifference, the CO₂ permeance R_(CO2) in Example 4 of Patent Document 3is calculated as 110 GPU. However, data with respect to CO₂/H₂selectivity in the present Example is not disclosed.

Patent Document 4 shown below discloses a CO₂ separation membrane formedof a cellulose acetate membrane containing an alkali bicarbonate addedtherein. However, Patent Document 4 describes only about CO₂/O₂selectivity and does not disclose data about CO₂/H₂ selectivity.Furthermore, the disclosed data are measured under the conditions of alow pressure (about 0.01 atm) and the data measured under the pressurecondition of about several atm are not disclosed.

-   Patent Document 1: Japanese Unexamined Patent Application    Publication (Translation of PCT Application) No. 2001-511430-   Patent Document 2: Specification of U.S. Pat. No. 6,579,331-   Patent Document 3: Japanese Unexamined Patent Application    Publication No. 2000-229219-   Patent Document 4: Specification of U.S. Pat. No. 3,396,510

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

Since the CO₂-facilitated transport membrane has a basic function ofselectively separating carbon dioxide, the development has been made forthe purpose of absorbing or removing carbon dioxide as the cause ofglobal warming. However, considering application of the CO₂-facilitatedtransport membrane to a CO₂ permeable membrane reactor, highperformances are required in the working temperature, CO₂ permeance andCO₂/H₂ selectivity. Since performances of a CO shift catalyst used forthe CO shift reaction tend to deteriorate with decreasing temperature,it is considered to be necessary that the working temperature is atleast 100° C. In any of Patent Documents 1 to 3, membrane performancesare measured under the temperature condition of about 25° C., and itcannot be said that the above patent documents disclosed theCO₂-facilitated transport membrane which exhibits sufficient membraneperformances even under the temperature of 100° C. or higher.

High CO₂ permeance (one of performance indicator of carbon dioxidepermeability) is required (for example, 2×10⁻⁵ mol/(m²·s·kPa) (=about 60GPU or more) so as to shift chemical equilibrium of the CO shiftreaction to the hydrogen production side (right side) and to decreasethe concentration of carbon monoxide and the concentration of carbondioxide to about 0.1% or less over limitation due to the restriction ofequilibrium, and to achieve higher performances (increase in SV) of theCO shift reaction. However, the CO₂ permeance of the CO₂-facilitatedtransport membrane described in Patent Documents 1 and 2 is the valuewhich is far less than 10 GPU, and the above patent documents do notdisclose the CO₂-facilitated transport membrane which exhibits the CO₂permeance of about 60 GPU or more.

Patent Document 3 does not disclose CO₂/H₂ selectivity, and also doesnot disclose that the CO₂ permeance is 60 GPU or more under thetemperature condition of 100° C. or higher. Patent Document 4 does notdisclose CO₂/H₂ selectivity, and also does not disclose data under thepressure condition of about several atm.

Furthermore, when hydrogen produced during the CO shift reaction isdischarged through the CO₂-facilitated transport membrane, together withcarbon dioxide, the process for separating and recovering hydrogen fromthe discharged gas is required. Since hydrogen has a molecular sizesmaller than that of carbon dioxide, the membrane, which enablespermeation of carbon dioxide, also enables permeation of hydrogen. Afacilitated transport membrane capable of selectively transporting onlycarbon dioxide using a carrier of carbon dioxide in the membrane isrequired and it is considered to be necessary that CO₂/H₂ selectivity isfrom about 90 to 100 or more.

However, the CO₂-facilitated transport membrane described in PatentDocuments 1 and 2 has a CO₂/H₂ selectivity of 19, and it cannot be saidto have sufficient selectivity. Since Patent Documents 3 and 4 do notdisclose CO₂/H₂ selectivity, it cannot be said that Patent Documents 3and 4 disclosed a CO₂-facilitated transport membrane which exhibits highCO₂/H₂ selectivity.

In light of the problems described above, an object of the presentinvention is to stably provide a CO₂-facilitated transport membranewhich can be applied to a CO₂ permeable membrane reactor.

Means for Solving the Problem

The CO₂-facilitated transport membrane according to the presentinvention for achieving the above object is characterized in that a gellayer in which an additive of cesium carbonate or cesium bicarbonate orcesium hydroxide is added to a polyvinyl alcohol-polyacrylic acidcopolymer gel membrane, is supported on the hydrophilic porous membrane.

According to the above characteristic of the CO₂-facilitated transportmembrane of the present invention, since cesium carbonate (Cs₂CO₃) iscontained in the polyvinyl alcohol-polyacrylic acid (PVA/PAA) copolymergel membrane, Cs₂CO₃ functions as a carbon dioxide carrier capable oftransporting carbon dioxide as a permeable material from the interfaceat the high carbon dioxide side of the PVA/PAA copolymer gel layer tothe interface at the low carbon dioxide side, thus making it possible toachieve a selectivity against hydrogen (CO₂/H₂) of about 50 or higher ata high temperature of 100° C. or higher, and a CO₂ permeance of about2×10⁻⁵ mol/(m²·s·kPa) (=60 GPU) or more.

Since a porous membrane supporting a PVA/PAA gel layer is hydrophilic, agel layer with fewer defects can be stably formed and high selectivityagainst hydrogen can be maintained. In general, when the porous membraneis hydrophobic, it is considered possible to prevent deterioration ofmembrane performances as a result of permeation of moisture in thePVA/PAA gel membrane into the pores of the porous membrane at 100° C. orlower. It is also considered that the similar effect can be expectedeven under the condition where the moisture content in the PVA/PAA gelmembrane decreases at 100° C. or higher. Therefore, use of a hydrophobicporous membrane is recommended. However, in the CO₂-facilitatedtransport membrane of the present invention, it became possible tostably produce a CO₂-facilitated transport membrane, which contain lessdefects and can maintain high selectivity against hydrogen, by using ahydrophilic porous membrane due to the following reasons.

When a cast solution which is an aqueous solution containing a PVA/PAAcopolymer and Cs₂CO₃ is cast on a hydrophilic porous membrane, the poresof the porous membrane are filled with the liquid, and also the castsolution is applied on the surface of the porous membrane. When thiscast solution is gelated, since not only the gel layer is formed on thesurface of the porous membrane, but also the pores are filled with thegel layer, defects are less likely to occur, resulting in high successrate of the formation of the gel layer.

Considering the proportion of pores (porosity) and considering thatpores are not straight vertically to the surface of the membrane, buttortuous (tortuosity), since the gel layer in pores becomes largeresistance to gas permeation, permeability becomes considerably low ascompared with that of the gel layer on the surface of the porousmembrane, the gas permeance decreases. On the other hand, when the castsolution is cast on a hydrophobic porous membrane, the pores of theporous membrane are not filled with the liquid, the cast solution isapplied only on the surface of the porous membrane with the pores filledwith gas. Therefore, it is presumed that the gas permeance in the gellayer on the hydrophobic porous membrane increases in both hydrogen andcarbon dioxide as compared with the hydrophilic porous membrane.

However, as compared with the gel layer in pores, microdefects arelikely to occur in the gel layer of the surface of the membrane and thusthe success rate of the formation of the membrane decreases. Sincehydrogen has a very small molecular size as compared with carbondioxide, the permeance of hydrogen remarkably increases as compared withcarbon dioxide at the microdefects. At the position other than defects,the permeance of carbon dioxide capable of permeating by a facilitatedtransporting mechanism is noticeably larger than that of hydrogencapable of permeating by a physical dissolution and diffusion mechanism.

As a result, selectivity against hydrogen (CO₂/H₂) when the hydrophobicporous membrane is used decreases as compared with the case where thehydrophilic porous membrane is used. In view of practical use, stabilityand durability of the CO₂-facilitated transport membrane become veryimportant. Therefore, it becomes advantageous to use a hydrophilicporous membrane having high selectivity against hydrogen (CO₂/H₂). Useof the hydrophilic porous membrane can be realized on the assumptionthat high CO₂ permeance can be achieved by adding Cs₂CO₃ as a carbondioxide carrier to a PVA/PAA gel layer.

The difference in gas permeance between the hydrophobic porous membraneand the hydrophilic porous membrane is estimated to similarly occur evenfor the case where a gel layer, which is prepared by gelation of a layerof a cast solution containing no Cs₂CO₃ as a CO₂ carrier, is impregnatedwith an aqueous Cs₂CO₃ solution since the situation that the gel layerin the pores has large resistance to gas permeation is the same for bothcases.

As described above, according to the CO₂-facilitated transport membranehaving the above characteristic, it becomes possible to provide aCO₂-facilitated transport membrane which can realize working temperatureof 100° C. or higher, CO₂ permeance of about 2×10⁻⁵ mol/(m²·s·kPa) (=60GPU) or more and CO₂/H₂ selectivity of about 90 to 100 or more, and canbe applied to a CO₂ permeable membrane reactor, and thus miniaturizationof the CO shift reactor, reduction of the start-up time and higherperformances (increase in SV) can be achieved.

The similar effect can also be obtained when cesium hydroxide is addedas the additive in place of cesium carbonate due to the followingreason. The reaction represented by Chemical Formula 2 shown below iscaused by using a facilitated transport membrane including a gel layercontaining cesium hydroxide added therein for separation of CO₂, therebyconverting cesium hydroxide added in the facilitated transport membraneinto cesium carbonate.CO₂+CsOH→CsHCO₃CsHCO₃+CsOH→Cs₂CO₃+H₂O  Chemical formula 2

Chemical formula 2 shown above can be summarized into Chemical formula 3shown below, which shows that cesium hydroxide added is converted intocesium carbonate.CO₂+2CsOH→Cs₂CO₃+H₂O  Chemical formula 3

Furthermore, as is apparent from Chemical formula 2 shown above, thesimilar effect can also be obtained by adding cesium bicarbonate as theadditive in place of cesium carbonate.

The CO₂-facilitated transport membrane according to the presentinvention has, in addition to the characteristic described above,another characteristic that the gel layer is formed such that a weightratio of cesium carbonate relative to the total weight of the polyvinylalcohol-polyacrylic acid copolymer gel membrane and cesium carbonate is65% by weight or more and 85% by weight or less.

According to the above characteristic of the CO₂-facilitated transportmembrane of the present invention, it becomes possible to provide aCO₂-facilitated transport membrane which can realize excellent CO₂permeance and excellent CO₂/H₂ selectivity under the temperaturecondition of 100° C. or higher, and can be applied to a CO₂ permeablemembrane reactor, and thus miniaturization of the CO shift reactor,reduction of the start-up time and higher performances (increase in SV)can be achieved.

The CO₂-facilitated transport membrane according to the presentinvention has, in addition to the above characteristic, anothercharacteristic that a gel layer in which an additive of rubidiumcarbonate or rubidium bicarbonate or rubidium hydroxide is added to apolyvinyl alcohol-polyacrylic acid copolymer gel membrane, is supportedon the hydrophilic porous membrane.

According to the above characteristic of the CO₂-facilitated transportmembrane of the present invention, rubidium carbonate (Rb₂CO₃) havingcomparatively high solubility in water functions as a CO₂ carrier in thepolyvinyl alcohol-polyacrylic acid (PVA/PAA) copolymer gel membrane,which transports carbon dioxide across the membrane from the interfaceof the high carbon dioxide concentration side to the interface of thelow carbon dioxide concentration side, thus making it possible toachieve a selectivity against hydrogen (CO₂/H₂) of about 90 to 100 ormore at a high temperature of 100° C. or higher, and a CO₂ permeance ofabout 2×10⁻⁵ mol/(m²·s·kPa) (=60 GPU) or more.

The similar effect can also be obtained by adding rubidium hydroxide orrubidium bicarbonate in place of rubidium carbonate. This is because ofthe same reason why the same effect, as that obtained by adding cesiumcarbonate, can be obtained by adding cesium hydroxide or cesiumbicarbonate in place of cesium carbonate.

The CO₂-facilitated transport membrane according to the presentinvention has, in addition to the above characteristic, anothercharacteristic that the gel layer supported on the hydrophilic porousmembrane is coated with a hydrophobic porous membrane.

According to the above characteristic of the CO₂-facilitated transportmembrane of the present invention, the gel layer supported on thehydrophilic porous membrane is protected by a hydrophobic porousmembrane and the strength of the CO₂-facilitated transport membraneincreases when in use. As a result, when the CO₂-facilitated transportmembrane is applied to a CO₂ permeable membrane reactor, sufficientmembrane strength can be ensured even when pressure difference at bothends (inside and outside the reactor) of the CO₂-facilitated transportmembrane increases (for example, 2 atm or more). Furthermore, since thegel layer is coated with the hydrophobic porous membrane, even whensteam is condensed on the surface of the hydrophobic porous membrane,permeation of water into the gel layer is prevented because the porousmembrane is hydrophobic. Therefore, the hydrophobic porous membraneprevents a carbon dioxide carrier in the gel layer from being dilutedwith water and the diluted carbon dioxide carrier from flowing out fromthe gel layer.

The CO₂-facilitated transport membrane according to the presentinvention has, in addition to the above characteristic, anothercharacteristic that the gel layer has an aldehyde group-derivedcross-linking structure.

According to the above characteristic of the CO₂-facilitated transportmembrane of the present invention, defects are less likely to occur inthe gel layer due to the cross-linking structure formed in the gellayer, resulting in drastic reduction of the H₂ permeance. On the otherhand, the CO₂ permeance does not drastically decrease as compared withthe H₂ permeance, thus making it possible to realize a facilitatedtransport membrane which exhibits high CO₂/H₂ selectivity.

The CO₂-facilitated transport membrane according to the presentinvention has, in addition to the above characteristic, anothercharacteristic that the hydrophilic porous membrane has heat resistanceat 100° C. or higher.

According to the above characteristic of the CO2-facilitated transportmembrane of the present invention, it becomes possible to use within awide temperature range from normal temperature to 100° C. or higher.Specifically, it becomes possible to use it under the temperature rangeof 100° C. or higher because the hydrophilic porous membrane has heatresistance of 100° C. or higher.

The CO₂-facilitated transport membrane according to the presentinvention has, in addition to the above characteristic, anothercharacteristic that both the gel layer and the hydrophilic porousmembrane have a cylindrical shape with the same central axis, and onemembrane is formed so as to bring an inner side face into contact withan outer side face of the other membrane thereby surrounding the othermembrane.

In this case, a membrane made of ceramics such as alumina can be used asthe hydrophilic porous membrane.

The gel layer can be formed outside the hydrophilic porous membrane soas to surround the hydrophilic porous membrane.

The method for producing the CO₂-facilitated transport membraneaccording to the present invention for achieving the above object ischaracterized by comprising the steps of preparing a cast solution whichis an aqueous solution containing a polyvinyl alcohol-polyacrylic acidcopolymer and cesium carbonate or cesium bicarbonate or cesiumhydroxide; and forming the gel layer by casting the cast solution on ahydrophilic porous membrane, and gelating the cast solution.

The method for producing the CO₂-facilitated transport membraneaccording to the present invention for achieving the above object hasanother characteristic that it comprises the steps of; preparing a castsolution which is an aqueous solution containing a polyvinylalcohol-polyacrylic acid copolymer and rubidium carbonate or rubidiumbicarbonate or rubidium hydroxide; and forming the gel layer by castingthe cast solution on a hydrophilic porous membrane and gelating the castsolution.

According to the above characteristic of the method for producing theCO₂-facilitated transport membrane of the present invention, since acast solution in which a mixing ratio of a carbon dioxide carrier to amembrane material (PVA/PAA) is properly adjusted is prepared in advance,optimization of a final mixing ratio of the carbon dioxide carrier inthe PVA/PAA gel membrane can be easily realized and improvement ofmembrane performances can be realized.

The method for producing the CO₂-facilitated transport membraneaccording to the present invention has, in addition to the abovecharacteristic, another characteristic that it further comprises a stepof forming a layered porous membrane in which a hydrophilic porousmembrane and a hydrophobic porous membrane are laid one upon anotherbefore the beginning of the step of forming the gel layer, wherein thestep of forming the gel layer includes a step of casting the castsolution on a surface of the hydrophilic porous membrane of the layeredporous membrane.

According to the above characteristic of the method for producing theCO₂-facilitated transport membrane of the present invention, it ispossible to realize a CO₂-facilitated transport membrane in which thegel layer supported on the hydrophilic porous membrane is protected by ahydrophobic porous membrane and the strength increases when in use.

The method for producing the CO₂-facilitated transport membraneaccording to the present invention has, in addition to the abovecharacteristic, another characteristic that the step of preparing thecast solution further includes a step of adding a cross-linking agenthaving an aldehyde group to a portion of a structure.

According to the above characteristic of the method for producing theCO₂-facilitated transport membrane of the present invention, since thecross-linking structure is formed in the membrane, defects are lesslikely to occur in the membrane resulting in drastic reduction of the H₂permeance, thus making it possible to realize a facilitated transportmembrane which exhibits high CO₂/H₂ selectivity.

In this case, glutaraldehyde or formaldehyde can be employed as across-linking agent to be added. Glutaraldehyde is added in an amount ofabout 0.008 to 0.015 g based on 1 g of a PVA/PAA copolymer, and thusparticularly high CO₂/H₂ selectivity can be exhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically showing a structure in oneembodiment of a CO₂-facilitated transport membrane according to thepresent invention.

FIG. 2 is a flow chart showing a method for producing a CO₂-facilitatedtransport membrane according to the present invention.

FIG. 3 is a sectional view schematically showing a structure of acomparative sample of a CO₂-facilitated transport membrane.

FIG. 4 is a block diagram of a test apparatus for evaluating membraneperformances of a CO₂-facilitated transport membrane according to thepresent invention.

FIG. 5 is a graph (1) showing the effect of improving CO₂/H₂ selectivityby use of a hydrophilic porous membrane in a CO₂-facilitated transportmembrane according to the present invention.

FIG. 6 is a graph (2) showing the effect of improving CO₂/H₂ selectivityby use of a hydrophilic porous membrane in a CO₂-facilitated transportmembrane according to the present invention.

FIG. 7 is a graph showing dependency of CO₂ permeance R_(CO2) and CO₂/H₂selectivity of a CO₂-facilitated transport membrane according to thepresent invention on the pressure of a feed gas and the carrierconcentration.

FIG. 8 is a graph showing dependency of CO₂ permeance R_(CO2) and CO₂/H₂selectivity of a CO₂-facilitated transport membrane according to thepresent invention on the carrier concentration.

FIG. 9 is a graph showing dependency of CO₂ permeance R_(CO2) and CO₂/H₂selectivity of a CO₂-facilitated transport membrane according to thepresent invention on the pressure of a feed gas and the workingtemperature.

FIG. 10 is a graph showing dependency of CO₂ permeance R_(CO2) andCO₂/H₂ selectivity of a CO₂-facilitated transport membrane according tothe present invention on the working temperature.

FIG. 11 is a graph showing dependency of CO₂ permeance R_(CO2) andCO₂/H₂ selectivity of a CO₂-facilitated transport membrane according tothe present invention on the pressure of a feed gas and the steam mol %.

FIG. 12 a graph showing the change with time of CO₂ permeance R_(CO2)and CO₂/H₂ selectivity of a CO₂-facilitated transport membrane accordingto the present invention.

FIG. 13 is a graph showing performances of a membrane of the presentinvention produced by a method of Example 1 of a second embodiment ofthe CO₂-facilitated transport membrane according to the presentinvention.

FIG. 14 is a graph showing performances of a membrane of the presentinvention produced by a method of Example 2 of a second embodiment ofthe CO₂-facilitated transport membrane according to the presentinvention.

FIG. 15 is a graph showing performances of a membrane of the presentinvention produced by a method of Example 3 of a second embodiment ofthe CO₂-facilitated transport membrane according to the presentinvention.

FIG. 16 is a graph showing the change with time of performances of amembrane of the present invention produced by a method of Example 1 of asecond embodiment of the CO₂-facilitated transport membrane according tothe present invention.

FIG. 17 is a sectional view schematically showing a structure of aCO₂-facilitated transport membrane of a third embodiment according tothe present invention.

FIG. 18 is a graph showing dependency of CO₂ permeance, H₂ permeance andCO₂/H₂ selectivity of a CO₂-facilitated transport membrane of a thirdembodiment according to the present invention on the temperature of afeed gas and the pressure.

FIG. 19 is a graph showing a comparison of the change with time of CO₂permeance R_(CO2) and CO₂/H₂ selectivity between cylinder type and flatplate type facilitated transport membranes.

FIG. 20 is a diagram showing flow of various gases in a CO shift reactorequipped with a CO₂-facilitated transport membrane.

FIG. 21 is a graph showing a comparison of the change in eachconcentration of carbon monoxide and carbon dioxide along the catalystlayer length of a CO shift reactor between whether the CO shift reactoris equipped with a CO₂-facilitated transport membrane or not.

BEST MODE FOR CARRYING OUT THE INVENTION

Each embodiment of the CO₂-facilitated transport membrane according tothe present invention and the method for producing the same will bedescribed with reference to the accompanying drawings.

First Embodiment

The first embodiment of the present invention will be described below.

The CO₂-facilitated transport membrane according to the presentinvention (hereinafter appropriately referred to as a “membrane of thepresent invention”) is a CO₂-facilitated transport membrane whichcontains a carbon dioxide carrier in a gel membrane containing moisture,and can be applied to a CO₂ permeable membrane reactor having a workingtemperature of 100° C. or higher, high carbon dioxide permeability andCO₂/H₂ selectivity. Furthermore, in the membrane of the presentinvention, a hydrophilic porous membrane is employed as a supportingmembrane for supporting a gel membrane containing a carbon dioxidecarrier so as to stably realize high CO₂/H₂ selectivity.

Specifically, in the membrane of the present invention, a polyvinylalcohol-polyacrylic acid (PVA/PAA) copolymer is used as a membranematerial and cesium carbonate (Cs₂CO₃) is used as a carbon dioxidecarrier. As schematically shown in FIG. 1, the membrane of the presentinvention has a three-layered structure in which a hydrophilic porousmembrane 2, on which a PVA/PAA gel membrane 1 containing a carbondioxide carrier is supported, is sandwiched between two hydrophobicporous membranes 3, 4. Hereinafter, the PVA/PAA gel membrane containinga carbon dioxide carrier is appropriately abbreviated to an “impregnatedgel membrane” in order to distinguish it from a PVA/PAA gel membranecontaining no carbon dioxide carrier and also from a membrane of thepresent invention having a structure equipped with two hydrophobicporous membranes. In the impregnated gel membrane, PVA/PAA exists in theproportion within a range from about 20 to 80% by weight and Cs₂CO₃exists in the proportion within a range from about 20 to 80% by weight,based on the total weight of PVA/PAA and Cs₂CO₃ in the impregnated gelmembrane.

The hydrophilic porous membrane preferably has, in addition tohydrophilicity, heat resistance at 100° C. or higher, mechanicalstrength, and tight adhesion with the impregnated gel membrane.Furthermore, the porosity is preferably within a range from 55% or more,and the pore diameter is preferably within a range from 0.1 to 1 μm. Inthe present embodiment, a hydrophilized polytetrafluoroethylene (PTFE)porous membrane is used as the hydrophilic porous membrane whichsatisfies these conditions.

The hydrophobic porous membrane preferably has, in addition tohydrophobicity, heat resistance at 100° C. or higher, mechanicalstrength and tight adhesion with the impregnated gel membrane.Furthermore, the porosity is preferably 55% or more and the porediameter is preferably within a range from 0.1 to 1 μm. In the presentembodiment, an unhydrophilized polytetrafluoroethylene (PTFE) porousmembrane is used as the hydrophobic porous membrane which satisfiesthese conditions.

One embodiment of the method for producing a membrane of the presentinvention (method of the present invention) will be described withreference to FIG. 2.

First, a cast solution which is an aqueous solution containing a PVA/PAAcopolymer and Cs₂CO₃ is prepared (step 1). More specifically, 1 g of aPVA/PAA copolymer (for example, manufactured by Sumitomo Seika ChemicalsCompany, Limited under the tentative name of SS gel) and 2.33 g ofCs₂CO₃ are charged in a sample bottle and 20 ml of water is added, andthen these components are dissolved by stirring them at room temperaturefor 5 days to obtain a cast solution.

Next, centrifugal separation (at a rotating speed of 5,000 rpm for 30minutes) is conducted so as to remove bubbles in the cast solutionobtained in the step 1 (step 2).

Next, the cast solution obtained in the step 2 is cast on the surface ofthe side of a hydrophilic PTFE porous membrane of a layered porousmembrane obtained by laying a hydrophilic PTFE porous membrane (forexample, manufactured by Sumitomo Electric Industries, Ltd., WPW-020-80,thickness: 80 μm, pore diameter: 0.2 μm, porosity: about 75%) on ahydrophobic PTFE porous membrane (for example, manufactured by SumitomoElectric Industries, Ltd., Fluoropore FP010, thickness: 60 μm, porediameter: 0.1 μm, porosity: 55%) using an applicator (step 3). A castthickness of the sample in Examples described hereinafter is 500 μm. Thecast solution permeates into pores in the hydrophilic PTFE porousmembrane. However, permeation stops at the boundary of the hydrophobicPTFE porous membrane and the cast solution does not permeate to theopposite surface of the layered porous membrane. Therefore, the castsolution does not exist on the side of the hydrophobic PTFE porousmembrane of the layered porous membrane and it becomes easy to handle.

After casting, the hydrophilic PTFE porous membrane is air-dried at roomtemperature for a day and the cast solution is gelated to form a gellayer (step 4). In the method of the present invention, since the castsolution is cast on the surface of the side of the hydrophilic PTFEporous membrane of the layered porous membrane in the step 3, the gellayer is not only formed on the surface (cast surface) of thehydrophilic PTFE porous membrane, but also formed in pores by fillingthereinto in the step 4. Therefore, defects (microdefects such aspinholes) are less likely to occur and the success rate of the formationof the gel layer increases. In the step 4, the air-dried PTFE porousmembrane is preferably thermally cross-linked at a temperature of about120° C. for about 2 hours. In samples of Examples and ComparativeExamples described hereinafter, any sample is thermally cross-linked.

Next, the same hydrophobic PTFE porous membrane as that of the layeredporous membrane used in the step 3 is laid on the side of the gel layerof the surface of the hydrophilic PTFE porous membrane obtained in thestep 4 to obtain a three-layered structure membrane of the presentinvention composed of a hydrophobic PTFE porous membrane/gel layer(impregnated gel membrane supported on the hydrophilic PTFE porousmembrane)/hydrophobic PTFE porous membrane as schematically shown inFIG. 1 (step 5). In FIG. 1, the state where pores of the hydrophilicPTFE porous membrane 2 are filled with an impregnated gel membrane 1 isschematically shown in a linear form.

The membrane of the present invention thus produced through the steps 1to 5 can realize membrane performances which can be applied to a CO₂permeable membrane reactor, that is, a working temperature of 100° C. orhigher, a CO₂ permeance of about 2×10⁻⁵ mol/(m²·s·kPa) (=60 GPU) or moreand a CO₂/H₂ selectivity of about 90 to 100 or more, as describedhereinafter.

By providing a three-layered structure in which the gel layer issandwiched between the hydrophobic PTFE porous membranes, onehydrophobic PTFE porous membrane is used in the steps 3 and 4 for thepurpose of supporting the hydrophilic PTFE porous membrane supportingthe impregnated gel membrane and preventing permeation of the castsolution, while the other hydrophobic PTFE porous membrane is used forthe purpose of protecting the impregnated gel membrane from the otherside.

Furthermore, even when steam is condensed on the surface of the membraneof the hydrophobic porous membrane, since the PTFE porous membrane ishydrophobic to repel water, permeation of water into the impregnated gellayer is prevented. Therefore, it is possible to prevent the carbondioxide carrier in the impregnated gel layer from being diluted withwater, and also to prevent the diluted carbon dioxide carrier fromflowing out of the impregnated gel layer.

Membrane performances of specific Examples will be described below.

First, the membrane composition of each sample of Examples in which ahydrophilic PTFE porous membrane is used as a porous membrane forsupporting an impregnated gel membrane, and Comparative Example in whicha hydrophobic PTFE porous membrane is used as a porous membrane will bedescribed below.

Samples of Examples are produced by the method described above. Themixing ratio of PVA/PAA: Cs₂CO₃ is 30% by weight: 70% by weight in thesequence of description. A proportion of the weight of a carrier basedon the total weight of a copolymer and a carrier is described as a“carrier concentration”. In the case of the above example, the carrierconcentration is 70% by weight (hereinafter referred to as “70% byweight”).

Each sample of Comparative Examples is produced by using asingle-layered hydrophobic PTFE porous membrane in place of the layeredporous membrane of a hydrophilic PTFE porous membrane and a hydrophobicPTFE porous membrane in the above method. Therefore, the sample ofComparative Examples is formed into a three-layered structure in which aPVA/PAA gel membrane 1 containing a carbon dioxide carrier is sandwichedbetween two hydrophobic porous membranes 3 and 4, as schematically shownin FIG. 3. The mixing ratio of PVA/PAA: Cs₂CO₃ is the same as inExamples.

The constitution and the test method for evaluating membraneperformances of each sample of Examples and Comparative Examples will bedescribed with reference to FIG. 4.

As shown in FIG. 4, each sample 10 is fixed between a feed side chamber12 and a permeation side chamber 13 of a flow type gas permeation cell11 (membrane area: 2.88 cm²) made of stainless steel using afluororubber gasket as a sealing material. A feed gas (mixed gas of CO₂,H₂ and H₂O) FG is fed to the feed side chamber 12 at a flow rate of2.24×10⁻² mol/min and a sweep gas (Ar gas) SG is fed to the permeationside chamber 13 at a flow rate of 8.18×10⁻⁴ mol/min. The pressure of thefeed side chamber 12 is adjusted by a back pressure regulator 15provided at the down stream side of a cold trap 14 along an exhaustpassage of an exhaust gas. The pressure of the permeation side chamber13 is atmospheric pressure. The gas composition after removing steam ina sweep gas SG′ discharged from the permeation side chamber 13 by a coldstrap 16 is quantitatively determined by a gas chromatograph 17, andpermeance [mol/(m²·s·Pa)] of CO₂ and H₂ are calculated from the gascomposition and the flow rate of Ar in the sweep gas SG′, and thenCO₂/H₂ selectivity is calculated by the ratio. A back pressure regulator19 is also provided between the cold trap 16 and the gas chromatograph17, and the pressure of the permeation side chamber 13 is adjusted bythe back pressure regulator.

In order to simulate the feed gas FG in a CO shift reactor, a mixed gasof CO₂, H₂ and H₂O is adjusted to a mixing ratio (mol %) of CO₂: 5.0%,H₂: 45% and H₂O: 50%. Specifically, a mixed gas having the above mixingratio is prepared by feeding water into mixed gas flow (flow rate at 25°C.: 200 cm³/min, 8.18×10⁻³ mol/min) of 10% CO₂ and 90% H₂ (mol %) usinga metering pump 18 (flow rate: 0.256 cm³/min, 1.42×10⁻² mol/min) andvaporizing moisture with heating to 100° C. or higher, and the resultantmixed gas is fed to the feed side chamber 12.

The sweep gas SG is fed so as to maintain a permeation driving force byreducing a partial pressure of the permeation side chamber of a gas tobe measured (CO₂, H₂) which permeates through a sample membrane, and agas (Ar gas) different from the gas to be measured is used.Specifically, an Ar gas (flow rate at 25° C.: 20 cm³/min, 8.13×10⁻⁴mol/min) is fed to the permeation side chamber 13.

In order to maintain working temperature of the sample membrane and thetemperatures of the feed gas FG and the sweep gas SG at a constanttemperature, the flow type gas permeation cell 11 to which the samplemembrane is fixed, and a preheating coil (not shown) for heating theabove gas are immersed in a constant-temperature bath set at apredetermined temperature.

Next, FIG. 5 and FIG. 6 show the results obtained by measuring CO₂permeance R_(CO2), H₂ permeance R_(H2) and CO₂/H₂ selectivity of eachsample of (1) Examples and (2) Comparative Examples in the state wherethe pressure of the feed gas FG (described as “the pressure at the feedside” on the graph, the same shall apply hereinafter) in the feed sidechamber 12 is applied within a range from 200 kPa to 400 kPa. FIG. 5shows the results obtained by measuring at a temperature of 160° C., andFIG. 6 shows the results obtained by measuring at a temperature of 180°C. As the value of the pressure at the feed side on the graph, the valueof the pressure of the back pressure regulator 15 for adjusting thepressure of the feed side chamber 12 is adopted.

As is apparent from FIG. 5 and FIG. 6, the H₂ permeance of the samplesusing the hydrophobic PTFE porous membrane of Comparative Examples ishigher than that of the samples using the hydrophilic PTFE porousmembrane of Examples in the entire pressure range, while CO₂ permeanceand CO₂/H₂ selectivity of the samples of Examples are remarkablyimproved as compared with the samples of Comparative Examples. Thisreason is considered as follows. That is, when the cast solution is caston the hydrophilic membrane, not only the gel layer is formed on thesurface of the PTFE porous membrane, but also pores are filled with thegel layer, defects (microdefects such as pinholes) are less likely tooccur and an increase in gas permeance, particularly H₂ permeancethrough the microdefects is suppressed. On the other hand, in the caseof the hydrophobic membrane, since the cast solution does not permeateinto pores of the membrane and is applied to the surface, defects arelikely to occur and the H₂ permeance increases, resulting indeterioration of selectivity.

As is apparent from FIG. 5 and FIG. 6, similar properties are exhibitedeven when the measuring temperature varies.

The CO₂-facilitated transport membranes disclosed in Patent Documents 1and 2 do not satisfy any of a working temperature of 100° C. or higher,a CO₂ permeance of about 2×10⁻⁵ mol/(m²·s·kPa) or more and a CO₂/H₂selectivity of about 90 to 100 or more, whereas, the samples of Examplesshown in FIG. 5 and FIG. 6 nearly satisfy all requirements within theentire pressure range. The samples of Comparative Examples also exhibita CO₂ permeance of about 2×10⁻⁵ mol/(m²·s·kPa) or more under a workingtemperature condition of 100° C. or higher. The samples of ComparativeExamples suggest that CO₂/H₂ selectivity drastically decreases when thepressure at the feed side is 300 kPa or more.

Considering the results of FIG. 5 and FIG. 6, as compared with theCO₂-facilitated transport membranes disclosed in Patent Documents 1 and2, the membranes equipped with a PVA/PAA gel membrane containing Cs₂CO₃of the present invention can improve CO₂ permeance under a hightemperature condition of 100° C. or higher. The values of CO₂ permeanceand CO₂/H₂ selectivity can be remarkably improved by using a hydrophilicporous membrane as a supporting membrane.

Similar to Examples, data are obtained using the membrane of the presentinvention, which has the constitution in which the impregnated gelmembrane is supported by the hydrophilic PTFE.

Next, FIG. 7 shows the results obtained by measuring CO₂ permeanceR_(CO2), H₂ permeance R_(H2) and CO₂/H₂ selectivity of each sample madeby varying the carrier concentration within a range from 50 to 85% byweight under the same conditions as in FIG. 5 of the mixing ratio andthe measurement temperature of the feed gas FG in the state where thepressure of the feed gas FG is within a range from 200 kPa to 600 kPa.

As is apparent from FIG. 7, CO₂ permeance R_(CO2) is maximized when thecarrier concentration is 70% by weight at a measuring temperature of160° C., and CO₂ permeance R_(CO2) is maximized when the pressure of thefeed gas FG is 500 kPa. It is also apparent that when the carrierconcentration is 65% by weight or more and 80% by weight or less, andwhen the carrier concentration is 85% by weight and the pressure of thefeed gas FG is 300 kPa or more, a high CO₂ permeance of 5.0×10⁻⁵mol/(m²·s·kPa) or more is exhibited.

It is also apparent that, the H₂ permeance R_(H2) usually tends toslightly decrease as the pressure of the feed gas FG entirely increased,with the exception of a carrier concentration of 50% by weight.

As is also apparent from FIG. 7, in case the carrier concentration is70% or more and 80% or less, a CO₂/H₂ selectivity of about 90 to 100 ormore is exhibited when the pressure of the feed gas FG is within a rangefrom 200 to 600 kPa.

From the results shown in FIG. 7, according to the membrane of thepresent invention, a working temperature of 100° C. or higher (160° C.),a CO₂ permeance of about 2×10⁻⁵ mol/(m²·s·kPa) (=60 GPU) or more and aCO₂/H₂ selectivity of about 90 to 100 or more can be realized byadjusting the carrier concentration. Therefore, the membrane of thepresent invention can be applied to a CO₂ permeable membrane reactor.

FIG. 8 shows a graph showing a relation between the carrierconcentration and the CO₂ permeance R_(CO2) and a relation betweencarrier concentration and the CO₂/H₂ selectivity when the feed gaspressure is constant (501.3 kPa). The mixing ratio of the feed gas FGand the measuring temperature are the same as those in the case of FIG.7.

As is apparent from FIG. 8, both CO₂ permeance and CO₂/H₂ selectivityshow the highest values when the carrier concentration is 70% by weight.In other words, as is apparent from FIG. 8, both CO₂ permeance andCO₂/H₂ selectivity depend on the carrier concentration. Particularly,when the membrane of the present invention is used as theCO₂-facilitated transport membrane, the ability can be exhibited as muchas possible by adjusting the carrier concentration to 70% by weight.

FIG. 9 shows the results obtained by measuring CO₂ permeance R_(CO2), H₂permeance R_(H2) and CO₂/H₂ selectivity under the conditions that thecarrier concentration is adjusted to 70% by weight and the mixing ratioof the feed gas FG is the same as in FIG. 7 when the measuringtemperature varies within a range of 125° C. or higher and 200° C. orlower in the state where the pressure of the feed gas FG in the feedside chamber 12 is within a range from 200 kPa to 600 kPa.

As is apparent from FIG. 9, the CO₂ permeance R_(CO2) becomes highestwhen the measuring temperature is 160° C. It is also apparent that theCO₂/H₂ selectivity is large when the measuring temperature is 160° C.and 180° C., and the CO₂/H₂ selectivity decreases even when thetemperature becomes higher or lower than the above temperature. In otherwords, as is apparent from FIG. 9, CO₂ permeance and CO₂/H₂ selectivityalso depend on the measuring temperature. Particularly, when themembrane of the present invention is used as the CO₂-facilitatedtransport membrane, the ability can be exhibited as much as possible byusing the membrane of the present invention under the temperaturecondition of 160° C. According to the membrane of the present invention,as compared with the conventional CO₂-facilitated transport membranesdisclosed in Patent Documents 1 and 2, high CO₂ permeance and highCO₂/H₂ selectivity can be realized under sufficiently high temperaturecondition (125° C. to 200° C.) and in particular, satisfactory valuescan be realized at 140° C. to 180° C.

Since the membrane of the present invention exhibits CO₂ permeanceR_(CO2) of about 1.0×10⁻⁴ mol/(m²·s·kPa) even when the measuringtemperature is 200° C., it is apparent that it exhibits a CO₂ permeanceof about 2×10⁻⁵ mol/(m²·s·kPa) or more. It is also apparent that thevalue of CO₂ permeance does not change very much under constanttemperature condition even when the pressure of the feed gas FG varies.

Furthermore, as is apparent from FIG. 9, the CO₂/H₂ selectivity exhibitsa value close to 100 under a pressure of 300 kPa under a hightemperature condition of 200° C. In other words, it is apparent that theCO₂-facilitated transport membrane, which can be applied to the CO₂permeable membrane reactor, can be realized even under a hightemperature condition of about 200° C.

FIG. 10 shows a relation between the measuring temperature and the CO₂permeance R_(CO2), and a relation between the measuring temperature andthe CO₂/H₂ selectivity when the feed gas pressure is constant (501.3kPa). The mixing ratio of the feed gas FG and the measuring temperatureare the same as those in the case of FIG. 9.

As is apparent from FIG. 10, both CO₂ permeance and CO₂/H₂ selectivityexhibit highest values when the measuring temperature is 160° C. Inother words, as is apparent from FIG. 10, both CO₂ permeance and CO₂/H₂selectivity depend on the measuring temperature. Particularly, when themembrane of the present invention is used as the CO₂-facilitatedtransport membrane, the ability can be exhibited as much as possible byusing the membrane of the present invention under a temperaturecondition of 160° C.

FIG. 11 shows the results obtained by measuring CO₂ permeance R_(CO2),H₂ permeance R_(H2) and CO₂/H₂ selectivity of the samples made byadjusting the carrier concentration to 70% by weight when the mixingratio of the feed gas FG and the measuring temperature are the same asthose in FIG. 6 and the steam mol % varies to 20%, 30%, 50%, 70% and 90%in the state where the pressure of the feed gas FG is within a rangefrom 200 kPa to 600 kPa. Specifically, the measurement is conducted byfixing CO₂ mol % of a mixture of CO₂, H₂ and H₂O to 5% and varying mol %of H₂ and mol % of H₂O (steam mol %) so as to adjust the total of mol %of H₂ and mol % of H₂O to 95%.

As is apparent from FIG. 11, the value of the CO₂ permeance increases assteam mol % increases, whereas, the value of the CO₂ permeance decreasesas steam mol % decreases. Even when steam mol % is decreased to about30%, the CO₂ permeance of about 1×10⁻⁴ mol/(m²·s·kPa) is exhibited undera pressure condition of the feed gas FG of 400 kPa.

The value of the H₂ permeance remarkably varies when steam mol % is 20%,but does not remarkably vary when steam mol % is the other value. It isapparent that the CO₂/H₂ selectivity entirely decreases as steam mol %decrease. Even when steam mol % is 30%, the CO₂/H₂ selectivity of about100 is exhibited under the pressure condition of the feed gas FG of 400kPa.

Therefore, as is apparent from the graph shown in FIG. 11, even underthe condition where steam mol % is set to the low value such as 30% orless, the membrane of the present invention exhibits excellentperformances and can realize a CO₂-facilitated transport membrane whichcan be applied to a CO₂ permeable membrane reactor.

FIG. 12 is a graph showing long-term performances of the membrane of thepresent invention. The graph shows the change with time of the values ofCO₂ permeance R_(CO2) and CO₂/H₂ selectivity when the feed gas isadjusted to a mixing ratio (mol %) of CO₂: 5%, H₂: 45% and H₂O: 50% andthe pressure of a feed gas is adjusted to 351.03 kPa, and the carrierconcentration is adjusted to 70% by weight.

As is apparent from FIG. 12, the value of the CO₂ permeance R_(CO2) doesnot remarkably vary with time, and exhibits the value of about 1.6×10⁻⁴mol/(m²·s·Pa). Further, the CO₂/H₂ selectivity does not remarkably varywith time, and exhibits a value of about 100. As described above,according to the membrane of the present invention, it is possible torealize a CO₂-facilitated transport membrane which does not causedrastic deterioration of performances with time, and also can be appliedto a CO₂ permeable membrane reactor which exhibits excellentperformances over a long period.

Table 1 described below shows a comparison of the values of CO₂permeance, H₂ permeance and CO₂/H₂ selectivity between the membrane ofthe present invention and the membranes in which the membrane materialis the same (PVA/PAA copolymer) and the materials used as a carbondioxide carrier are various carbonates other than Cs₂CO₃. Table 1 showsdata obtained by measuring the above values when carbonates of Na, K andRb are used as the carbon dioxide carrier, in addition to the carbonateof Cs used in the membrane of the present invention. In any case, dataare obtained by adjusting the feed gas pressure to 401.33 kPa, themeasuring temperature to 160° C. and the feed gas to a mixing ratio (mol%) of CO₂: 5.0%, H₂: 45% and H₂O: 50%. Each membrane is produced by thesame method as that of the membrane of the present invention.

TABLE 1 Solubility in Concentration water [g/100 of carbonate in CO₂ H₂g − water membrane permeance permeance CO₂/H₂ Carbonate (temperature)][% by weight] [mol/m² · s · kPa] [mol/m² · s · kPa] selectivity Na₂CO₃29.4 (25° C.) 34.6 3.03 × 10⁻⁶ 6.89 × 10⁻⁶ 0.44 K₂CO₃ 112.1 (25° C.)49.7 1.00 × 10⁻⁴ 1.92 × 10⁻⁶ 52.5 Rb₂CO₃ 450 (20° C.) 62.3 1.16 × 10⁻⁴2.75 × 10⁻⁶ 52.3 Rb₂CO₃ 450 (20° C.) 70 1.21 × 10⁻⁴ 5.54 × 10⁻⁷ 219Cs₂CO₃ 260.5 (15° C.) 70 1.90 × 10⁻⁴ 1.53 × 10⁻⁶ 125

As is apparent from the results shown in Table 1, in the case of theNa₂CO₃ membrane, very low CO₂ permeance and high H₂ permeance areexhibited. This reason is considered as follows. That is, since Na₂CO₃has low solubility in water (see Table 1), a crystal is produced whenthe cast membrane is cross-linked at 120° C. and thus a uniform membranecannot be obtained. In the case of the K₂CO₃ membrane, although high CO₂permeance is obtained, defects are likely to occur in the membrane, andthus H₂ permeance increases and high CO₂/H₂ selectivity cannot beobtained. In the case of the membrane containing Rb₂CO₃ and Cs₂CO₃ eachhaving high solubility in water (see Table 1), satisfactory CO₂permeance and CO₂/H₂ selectivity are obtained.

As described above, it became apparent that carbonates having highsolubility in water efficiently functions as a CO₂ carrier even at hightemperature and the membrane containing the same is less likely to causedefects and exhibits high CO₂ permeability and selectivity. The membraneof the present invention using Cs₂CO₃ as the carrier can realize aCO₂-facilitated transport membrane which exhibits high CO₂ permeance andhigh CO₂/H₂ selectivity.

Second Embodiment

The second embodiment of the present invention will be described below.Since the present embodiment differs from the first embodiment only inpartial constitution of the membrane of the present invention and themethod of the present invention, repetitive descriptions of the sameconstituent element are omitted.

The present embodiment differs from the first embodiment in the contentsof the step of preparing a cast solution (step 1 described above). Inthe present embodiment, the following three steps are conducted as thestep corresponding to the step 1 of the first embodiment (step ofpreparing a cast solution) and are referred to as Examples 1 to 3,respectively.

EXAMPLE 1

First, 20 g of water is added to 1 g of a PVA/PAA copolymer (forexample, manufactured by Sumitomo Seika Chemicals Company, Limited underthe tentative name of SS gel) and then a gel is dissolved by stirring atroom temperature. To the solution, about 0.008 to 0.0343 g ofglutaraldehyde is added, followed by stirring it under a temperaturecondition of 95° C. for 15 hours. To the solution, 2.33 g of Cs₂CO₃ isadded, followed by stirring it at room temperature to obtain a castsolution. In Example 1, the cast solution is prepared by conducting agel dissolution step, a glutaraldehyde addition step, a stirring step ata high temperature, a Cs₂CO₃ addition step and a stirring step at roomtemperature in this order.

EXAMPLE 2

First, 20 g of water is added to 1 g of a PVA/PAA copolymer and then agel is dissolved by stirring it at room temperature. To the solution,2.33 g of Cs₂CO₃ and 0.008 to 0.0343 g of glutaraldehyde are added anddissolved by stirring the solution at room temperature. Then, thesolution is stirred under a temperature condition of 95° C. for 15 hoursto obtain a cast solution. In Example 2, the cast solution is preparedby conducting a gel dissolution step, a glutaraldehyde and Cs₂CO₃addition step, a stirring step at room temperature and a stirring stepat a high temperature in this order.

EXAMPLE 3

First, 20 g of water is added to 1 g of a PVA/PAA copolymer and then agel is dissolved by stirring it at room temperature. To the solution,2.33 g of Cs₂CO₃ and 0.008 to 0.0343 g of glutaraldehyde are added anddissolved by stirring the solution at room temperature to obtain a castsolution. In Example 3, the cast solution is prepared by conducting agel dissolution step, a glutaraldehyde and Cs₂CO₃ addition step and astirring step at room temperature in this order.

In any of Examples 1 to 3, after preparing the cast solution, aCO₂-facilitated transport membrane is obtained by using the same methodas in the steps (step 2 to 4) described in the first embodiment. Aftercentrifugal separation is conduced so as to remove bubbles in the castsolution, the above cast solution is cast on the surface of the side ofa hydrophilic PTFE porous membrane of a layered porous membrane, whichis obtained by laying a hydrophobic PTFE porous membrane (thickness: 60μm) and a hydrophilic PTFE porous membrane (thickness: 80 μm) one uponanother on a glass plate, in a thickness of 500 μm using an applicator.Then, the cast solution is dried at room temperature for a day. ACO₂-facilitated transport membrane is obtained by maintaining under ahigh temperature condition of about 120° C. for 2 hours.

Membrane performances of the membranes of the present invention producedby the methods of Examples 1 to 3 will be described below. Regarding themembrane composition, the carrier concentration is adjusted to 70% byweight similarly to Examples of the first embodiment, and a testapparatus and a test method for evaluation of membrane performances arealso the same as those in the first embodiment.

FIG. 13 shows the results obtained by measuring (a) CO₂ permeanceR_(CO2), (b) H₂ permeance R_(H2) and (c) CO₂/H₂ selectivity of themembranes of the present invention produced using the cast solutionprepared by the method of Example 1 in the state where the pressure atthe feed side is within a range from 200 kPa to 600 kPa. In FIG. 13,data are measured by varying the amount of glutaraldehyde added in thecast solution. The test is conducted in three patterns by using (1)0.008 g, (2)0.0153 g, (3) 0 g (no addition) as the additive amount ofglutaraldehyde. On the graph, glutaraldehyde is abbreviated to “GA” (thesame shall apply to the graphs shown below).

Test conditions are as follows: the temperature condition: 160° C., feedgas FG: mixing ratio (mol %) of CO₂: 5.0%, H₂:45% and H₂O: 50%, flowrate of feed gas FG: 360 cm³/min at 25° C. under 1 atm, pressure at thepermeation side is 20 kPa lower than the pressure at the feed side, andflow rate of sweep gas SG: 40 cm³/min at 25° C. under 1 atm. These testconditions are the same in the respective Examples.

In FIG. 13A, when glutaraldehyde is added, the CO₂ permeance R_(CO2)slightly decreases as compared with the case where glutaraldehyde is notadded. However, as is apparent from FIG. 13B, since the H₂ permeanceR_(H2) drastically decreases when glutaraldehyde is added, the CO₂/H₂selectivity is remarkably increased by adding glutaraldehyde as comparedwith the case where glutaraldehyde is not added, as shown in FIG. 13C.The reason for this is considered as follows. That is, since across-linking structure is formed by adding glutaraldehyde, defects ofthe membrane are less likely to occur and thus the H₂ permeance isremarkably decreased. As is apparent from FIG. 13B and FIG. 13C, when0.008 g of glutaraldehyde is added, the H₂ permeance is low and theCO₂/H₂ selectivity is high as compared with the case where 0.0153 g ofglutaraldehyde is added. As a result, it is suggested that selectivitydoes not become higher as the additive amount of glutaraldehydeincreases, and a proper additive amount capable of realizing highselectivity exists according to the test conditions.

FIG. 14 shows the results obtained by measuring (a) CO₂ permeanceR_(CO2), (b) H₂ permeance R_(H2) and (c) CO₂/H₂ selectivity of themembranes of the present invention produced using the cast solutionprepared by the method of Example 2 in the state where the pressure atthe feed side is within a range from 200 kPa to 600 kPa. In FIG. 14,data are obtained by varying the amount of glutaraldehyde added in thecast solution. The test is conducted in three patterns by using (1)0.008 g, (2) 0.0165 g, (3) 0 g (no addition) as the additive amount ofglutaraldehyde. Other test conditions are the same as those in Example1.

In FIG. 14A, when glutaraldehyde is added, the CO₂ permeance R_(CO2)slightly decreases as compared with the case where glutaraldehyde is notadded, similar to FIG. 13A. As is apparent from FIG. 14B, since the H₂permeance R_(H2) drastically decreases when glutaraldehyde is added,similar to FIG. 13B, the CO₂/H₂ selectivity is remarkably increased byadding glutaraldehyde as compared with the case where glutaraldehyde isnot added as shown in FIG. 14C. The reason for this is considered to bethe same reason as in the case of Example 1. That is, since across-linking structure is formed by adding glutaraldehyde, defects ofthe membrane are less likely to occur and thus the H₂ permeance isremarkably decreased. As is apparent from FIG. 14B and FIG. 14C, when0.008 g of glutaraldehyde is added, the H₂ permeance is low and theCO₂/H₂ selectivity is high as compared with the case where 0.0165 g ofglutaraldehyde is added. As a result, it is suggested that selectivitydoes not become higher as the additive amount of glutaraldehydeincreases, and a proper additive amount capable of realizing highselectivity exists according to the test conditions. In FIG. 14C, in therange where the gas pressure at the feed side is high, the difference inselectivity due to the amount of glutaraldehyde added decreases.

FIG. 15 shows the results obtained by measuring (a) CO₂ permeanceR_(CO2), (b) H₂ permeance R_(H2) and (c) CO₂/H₂ selectivity of themembranes of the present invention produced using the cast solutionprepared by the method of Example 3 in the state where the pressure atthe feed side is within a range from 200 kPa to 600 kPa. In FIG. 14,data are measured by varying the amount of glutaraldehyde added in thecast solution. The test is conducted in four patterns by using (1) 0.008g, (2) 0.0154 g, (3) 0.0343 g and (4) 0 g (no addition) as the additiveamount of glutaraldehyde. Other test conditions are the same as those inExample 1.

In FIG. 15A, when glutaraldehyde is added, the CO₂ permeance R_(CO2)slightly decreases as compared with the case where glutaraldehyde is notadded, similar to FIG. 13A. As is apparent from FIG. 15B, since the H₂permeance R_(H2) drastically decreases when glutaraldehyde is added,similar to FIG. 13B, the CO₂/H₂ selectivity is remarkably increased byadding glutaraldehyde as compared with the case where glutaraldehyde isnot added as shown in FIG. 15C. The reason for this is considered to bethe same reason as in the case of Example 1. That is, since across-linking structure is formed by adding glutaraldehyde, defects ofthe membrane are less likely to occur and thus the H₂ permeance isremarkably decreased. As is apparent from FIG. 15B and FIG. 15C, when0.008 g of glutaraldehyde is added, the H₂ permeance is low and theCO₂/H₂ selectivity is high as compared with the case where 0.0154 g ofglutaraldehyde is added and the case where 0.0343 g of glutaraldehyde isadded. As a result, it is suggested that selectivity does not becomehigher even if the additive amount of glutaraldehyde increases, and aproper additive amount capable of realizing high selectivity existsaccording to the test conditions.

Further, in FIG. 15C, in the range where the gas pressure at the feedside is high, the difference in selectivity due to the amount ofglutaraldehyde added decreases.

Referring to each graph of FIG. 13 to FIG. 15, by cross-linking the gelmembrane with glutaraldehyde, it becomes possible to remarkably reducethe H₂ permeability without so much deterioration of CO₂ permeability ascompared with the case where glutaraldehyde is not added, thus making itpossible to realize a facilitated transport membrane which exhibits highCO₂/H₂ selectivity. Particularly, when about 0.008 to 0.015 g ofglutaraldehyde is added to 1 g of the PVA/PAA copolymer (hereinafter,such a range is referred to “satisfactory range”), the CO₂/H₂selectivity is remarkably improved.

In Examples 1 to 3, there is not a remarkable difference in membraneperformances. That is, even when the membrane is produced by any method,the effect of improving the CO₂/H₂ selectivity by the addition ofglutaraldehyde can be realized. Particularly in Examples 2 and 3, evenwhen the gel membrane is cross-linked with glutaraldehyde, a decrease inCO₂ permeance is suppressed. In Example 1, even when the pressure at thefeed side increases, an increase in H₂ permeance is limited.

FIG. 16 is a graph showing long-term performances when glutaraldehyde isadded. Specifically, the graph shows the change with time of (a) CO₂permeance R_(CO2), (b) H₂ permeance R_(H2) and (c) CO₂/H₂ selectivitywhen a long-term test is conducted using the membranes (additive amountof glutaraldehyde: 0.0339 g) produced by the method of Example 1. Thepressure at the feed side is adjusted to 401.3 kPa, and other testconditions are the same as those in FIG. 13 to FIG. 15.

The test method is as follows. The membrane of the present invention isset to a permeation cell at about 10 AM and the temperature is raised to160° C., and then a feed gas and a sweep gas are fed, followed by apermeation test. The test is continued under the same conditions untilaround 8 PM. At around 8 PM, the feed gas is stopped and the temperatureis lowered to room temperature. At around 10 AM in the next morning, thesimilar test is conducted using the same membrane without decomposingthe permeation cell. Such a test is repeatedly continued for 2 weeks.The results are shown in FIG. 16A to FIG. 16C.

Regarding test data of FIG. 16, since the amount of glutaraldehyde addedis slightly more than the satisfactory range, the CO₂ permeance shows asmall value as compared with the values of FIG. 13 to FIG. 15. However,the H₂ permeance shows a drastically small value as compared with thecase where glutaraldehyde is not added even after a lapse of time, andalso CO₂/H₂ selectivity maintains a high value of 200 or more. Like thepresent evaluation method, when evaluation with a lapse of time isconducted by repeating start-up and shut-down, since variation intemperature (room temperature to 160° C.) and variation in pressure(normal pressure to 6 atm) are repeatedly applied to the membrane, theload on the membrane is increased compared to continuing the test at thesame temperature under the same pressure where long-term performancesare evaluated. In FIG. 16, since membrane performances are stable forabout 2 weeks even by the present evaluation method for repeatingstart-up and shut-down, it can be said that stability of the membranecan be remarkably improved by adding glutaraldehyde.

Although glutaraldehyde is employed as the material to be added in thepresent embodiment, since the addition step of the material is conductedin order to form a cross-linking structure in the membrane, the materialis not limited to glutaraldehyde as long as it is the material capableof forming the cross-linking structure. When the cross-linking structureis formed by an aldehyde group, for example, formaldehyde can also beused. Even when the material used as a carbon dioxide carrier is amaterial other than Cs₂CO₃ (for example, Rb₂CO₃), membrane performancescan be further improved by similarly introducing an additive to form across-linking structure.

Third Embodiment

The third embodiment of the present invention will be described below.The present embodiment differs from the first and second embodiments inthe shape of the membrane of the present invention.

In the above first and second embodiments, a description is made on theassumption of a facilitated transport membrane having a flat plate typestructure as shown in FIG. 1. In contrast, in the present embodiment, adescription is made on the assumption of a facilitated transportmembrane having a cylindrical shape as shown in FIG. 17.

FIG. 17 is a schematic view showing a structure of a facilitatedtransport membrane of the present embodiment. FIG. 18 is a graph showingCO₂ permeance, H₂ permeance and CO₂/H₂ selectivity when a facilitatedtransport membrane having a cylindrical shape is used.

FIG. 17A is a sectional view cut in parallel to a horizontal plane, andFIG. 17B is a sectional view cut in a direction vertical to a horizontalplane. The facilitated transport membrane shown in FIG. 17 has astructure in which a gel membrane 41 containing a carrier is supportedon an outer periphery of a cylindrical hydrophilic supporting membranemade of ceramics 42. In the present embodiment, the same gel membrane 41made from a cast solution as in the first embodiment is used. That is,Cs₂CO₃ is used as a carrier and is thermally cross-linked. As ceramics,for example, alumina can be used.

As shown in FIG. 17, a space 40 is provided between the gel membrane 41and an outer frame, and also a space 43 is provided inside thesupporting membrane made of ceramics 42.

In the evaluation of membrane performances, the same feed gas FG as inthe above embodiment is allowed to flow into a space 40, while an inertsweep gas SG is allowed to flow into a space 43. A portion of the feedgas FG allowed to flow into the space 40 permeates through a gelmembrane 41 containing a carrier (and supporting membrane 42) and isallowed to flow into the space 43 as a permeate gas PG. An inert sweepgas SG for discharging this permeate gas PG out of the system is allowedto flow into the space 43, and an exit gas SG′ as a mixture of thissweep gas SG and a permeate gas PG is fed into a cold trap 16 shown inFIG. 4. The method for calculating the permeance and selectivity is thesame as in the first embodiment.

FIG. 18 is a graph based on data obtained when the cylindricalfacilitated transport membrane shown in FIG. 17 is used as a facilitatedtransport membrane and the measuring method, carrier concentration andfeed gas pressure are the same as those in FIG. 9, and the measuringtemperature is set to 160° C. Similar to FIG. 9, both CO₂ permeance andCO₂/H₂ selectivity exhibit high values and it is apparent that thecylinder type facilitated transport membrane having a structure shown inFIG. 17 exerts the same effect as that of the flat plate type one shownin FIG. 1.

The structure shown in FIG. 17 has the constitution in which a gelmembrane 41 is exposed in a space 40 so that a gel membrane 41 isdirectly contacted with a feed gas FG. As compared with the structureshown in FIG. 1, the gel membrane 41 is not coated with a hydrophobicmembrane. This hydrophobic membrane has the effect of stabilizing thegel membrane and suppressing deterioration of performances with time.However, as shown in FIG. 19, a cylindrical facilitated transportmembrane has the effect of improving performances with time withoutbeing coated with a hydrophobic membrane. The respect will be describedbelow.

FIG. 19 is a graph in which long-term performances of flat plate typeand cylinder type facilitated transport membranes are compared, FIG. 19Ashows CO₂ permeance R_(CO2), and FIG. 19B shows CO₂/H₂ selectivity. Inany graph, (1) shows data of a cylinder type one, (2) shows data of aflat plate type one. The conditions used when the results are obtainedare the same as those in FIG. 12.

In FIG. 19, as a flat plate type facilitated transport membrane ofComparative Example, a facilitated transport membrane having a structurein which a gel membrane is not coated with hydrophobic membrane isassumed due to the following reason. Since the cylindrical membrane isin the state where one surface of the gel membrane is exposed to a feedgas, the conditions of the flat plate type one are made identical.

In FIG. 19A, the CO₂ permeance does not remarkably vary with time inboth flat plate type and cylinder type. In FIG. 19B, the CO₂/H₂selectivity does not remarkably vary with time in the case of a cylindertype facilitated transport membrane, whereas, the selectivitydeteriorates with time and deteriorates to about 10% of the maximumafter a lapse of 100 hours in the flat plate type facilitated transportmembrane. As a result, it is considered that when the gel membrane isnot coated with the hydrophobic membrane, the cylindrical facilitatedmembrane is superior to the flat plate type one in view of long-termperformances. As is apparent from FIG. 12 and FIG. 16, satisfactorylong-term performances are exhibited by coating the gel membrane withthe hydrophobic membrane in the flat plate type one.

It is preferred that the supporting membrane made of ceramics used inthe present embodiment has a heat resistance of 100° C. or higher,mechanical strength and tight adhesion with the impregnated gelmembrane, similar to the case of the PTFE porous membrane described inthe first embodiment. The porosity is preferably 40% or more and thepore diameter is preferably within a range from 0.1 to 1 μm.

With the constitution of FIG. 17, the structure is provided with asupporting membrane made of ceramics formed inside and a gel membraneformed outside the supporting membrane. To the contrary, the supportingmembrane may be formed outside and the gel membrane may be formed insidethe supporting membrane. It is described that the shape is“cylindrical”. However, this description does not necessarily requirethat the cross section has a precise circle shape and may be an ovalshape, or may have slight unevenness.

According to the present embodiment, it is shown that long-termperformances are improved by using a cylindrical facilitated membrane ascompared with a flat plate type one. This reason is considered that thefacilitated transport membrane is less likely to be deformed and alsostabilized by using a cylindrical shape. In the case of a flat platetype one, it is considered that defects occur as a result of deformationof the membrane with time, and selectivity deteriorates as a result ofleakage of H₂ from the defects. While a ceramic membrane is used as asupporting membrane in the above Examples, this membrane can be formedinto a cylindrical shape and the material is not limited to ceramics aslong as it is the material which is less likely to be deformed withtime.

While a PTFE porous membrane is used as a supporting membrane in thefirst and second embodiments, the membrane is not limited to the PTFEporous membrane as long as the flat plate state can be maintainedwithout being broken in the state where the pressure is applied.

Another embodiment will be described below.

(1) In the above respective embodiments, the membrane of the presentinvention is produced by casting a cast solution which is an aqueoussolution containing a PVA/PAA copolymer and Cs₂CO₃ as a carbon dioxidecarrier on a hydrophilic PTFE porous membrane for gel membranesupporting, and gelating the cast solution. However, the membrane of thepresent invention may be produced by the method other than this method.For example, the membrane of the present invention may be produced byimpregnating a PVA/PAA copolymer gel membrane with an aqueous Cs₂CO₃solution.

(2) While the case of producing a CO₂-facilitated transport membrane byadding cesium carbonate as an additive to a gel membrane is described inthe above first embodiment, the same effect can be obtained even whencesium hydroxide is used in place of cesium carbonate. This reason is asfollows. That is, the reaction represented by Chemical formula 2 shownabove is caused by using a gel membrane containing cesium hydroxideadded therein to CO₂ separation, thereby converting cesium hydroxideinto cesium carbonate. Furthermore, even when cesium bicarbonate is usedin place of cesium carbonate, the same effect can be obtained byChemical formula 2 shown above.

Similarly, even when a CO₂-facilitated transport membrane is produced byadding rubidium carbonate as an additive to a gel membrane, rubidiumhydroxide or rubidium bicarbonate can be used in place of rubidiumcarbonate.

(3) While the membrane of the present invention has a three-layeredstructure of hydrophobic PTFE porous membrane/gel layer (impregnated gelmembrane supported on hydrophilic PTFE porous membrane)/hydrophobic PTFEporous membrane in the above embodiment, the supporting structure of themembrane of the present invention is not necessarily limited to thethree-layered structure. For example, a two-layered structure ofhydrophobic PTFE porous membrane/gel layer (impregnated gel membranesupported on hydrophilic PTFE porous membrane) may be used.

(4) While the case of applying the membrane of the present invention toa CO₂ permeable membrane reactor was assumed in the above embodiment,the membrane of the present invention can be used for the purpose ofselectively separating carbon dioxide, in addition to the CO₂ permeablemembrane reactor. Therefore, the feed gas to be fed to the membrane ofthe present invention is not limited to the mixed gas exemplified in theabove embodiment.

(5) The mixing ratio in the composition of the membrane of the presentinvention, and the size of each portion of the membrane in the aboveembodiment are exemplified for easier understanding of the presentinvention, and the present invention is not limited to theCO₂-facilitated transport membrane of these numerical values.

Industrial Applicability

The CO₂-facilitated transport membrane according to the presentinvention can be used to separate carbon dioxide, and particularly itcan be used as a CO₂-facilitated transport membrane which can separatecarbon dioxide contained in a reformed gas for a fuel cell, containinghydrogen as a main component with high selectivity over hydrogen, and isalso for a CO₂ permeable membrane reactor.

EXPLANATION OF REFERENCES 1 PVA/PAA gel membrane (gel layer) containingcarbon dioxide carrier 2 Hydrophilic porous membrane 3, 4 Hydrophobicporous membrane 10 CO₂-facilitated transport membrane (sample) 11 Flowtype gas permeation cell 12 Feed side chamber 13 Permeation side chamber14, 16 Cold trap 15 Back pressure regulator 17 Gas chromatograph 18Metering pump 19 Back pressure regulator 40 Space 41 Gel membrane 42Supporting membrane made of ceramics 43 Space FG Feed gas SG, SG′ Sweepgas

1. A method for producing a CO₂-facilitated transport membrane havingCO₂/H₂ selectivity under a temperature condition of 100° C., or higher,the method comprising steps of: preparing a cast solution which is anaqueous solution containing a polyvinyl alcohol-polyacrylic acidcopolymer and at least one of cesium carbonate and cesium bicarbonateand cesium hydroxide; and forming a gel layer by casting the castsolution on a hydrophilic porous membrane having a heat resistance of100° C. or higher, and gelating the cast solution.
 2. The method forproducing a CO₂-facilitated transport membrane according to claim 1, themethod further comprising a step of: forming a layered porous membranein which a hydrophilic porous membrane and a hydrophobic porous membraneare laid one upon another before beginning of the step of forming thegel layer, wherein: the step of forming the gel layer includes a step ofcasting the cast solution on a surface of the hydrophilic porousmembrane of the layered porous membrane.
 3. The method for producing aCO₂-facilitated transport membrane according to claim 1, wherein: thestep of preparing the cast solution further includes a step of adding across-linking agent having an aldehyde group to a portion of astructure.
 4. A method for producing a CO₂-facilitated transportmembrane having CO₂/H₂ selectivity under a temperature condition of 100°C. or higher, the method comprising steps of: preparing a cast solutionwhich is an aqueous solution containing a polyvinyl alcohol-polyacrylicacid copolymer and at least one of rubidium carbonate and rubidiumbicarbonate and rubidium hydroxide; and forming a gel layer by castingthe cast solution on a hydrophilic porous membrane having a heatresistance of 100° C. or higher, and gelating the cast solution.
 5. Themethod for producing a CO₂-facilitated transport membrane according toclaim 4, the method further comprising the step of: forming a layeredporous membrane in which a hydrophilic porous membrane and a hydrophobicporous membrane are laid one upon another before beginning of the stepof forming the gel layer, wherein: the step of forming the gel layerincludes a step of casting the cast solution on a surface of thehydrophilic porous membrane of the layered porous membrane.
 6. Themethod for producing a CO₂-facilitated transport membrane according toclaim 4, wherein: the step of preparing the cast solution furtherincludes a step of adding a cross-linking agent having an aldehyde groupto a portion of a structure.
 7. The method for producing aCO₂-facilitated transport membrane according to claim 1, wherein theCO₂/H₂ selectivity under a temperature condition of 100° C. or higher is90 or higher.
 8. The method for producing a CO₂-facilitated transportmembrane according to claim 4, wherein the CO₂/H₂ selectivity under atemperature condition of 100° C. or higher is 90 or higher.